Rate limiting middleware

[bpamiri]

Phase 2 — Modernization Target #13

Add built-in rate limiting as a middleware component.

Design

• Ship wheels.middleware.RateLimiter (depends on middleware pipeline, Middleware pipeline #1906)
• Configurable per-route or per-group limits
• Support multiple strategies: fixed window, sliding window, token bucket
• Storage backends: in-memory (single server), database (multi-server)
• Return standard 429 Too Many Requests with Retry-After header

Dependencies

Requires the middleware pipeline (#1906) to be implemented first.

See design_docs/MODERNIZE-WHEELS-RIM.md §5 (Phase 2) for context.